Throughout this document, the term interstitial elements refers to those atoms that, because of their small size with respect to the main elements in the alloy, are able to diffuse interstitially, that is, via the spaces in the metallic crystalline lattice, without the need to displace other atoms from their positions in the lattice. In the case of many alloys, like steel, atoms like hydrogen, nitrogen carbon and others can act like interstitial elements.
It is known that hydrogen is an interstitial element that can cause the embrittlement of steel components. Specifically, the sensitivity to hydrogen embrittlement is more evident in high-strength alloys.
Various mechanisms have been described as responsible for said embrittlement. These mechanisms do not begin to materialize as long as the temperature does not drop below a given threshold so that the interstitial elements in question feature a reduced mobility and an insufficient solubility, and tend to combine with other elements to form embrittling compounds.
It is known that hydrogen features a solubility which varies from one metallurgical phase to another and at the same time, solubility increases within each phase as temperature increases. For example, in the case of the solid phases of steel, hydrogen solubility ranges between 8 ppm in high temperature austenite (1400° C.), and less than 1 ppm in room temperature ferrite, and it is approximately 30 ppm in the liquid phase at 1600° C.
It can be considered that the phenomenon of diffusion of interstitial elements is governed mainly by the interstitial atom's thermal agitation within the crystalline lattice, i.e., at higher temperatures, greater thermal agitation and, therefore, greater probability of diffusion. Although the situation usually considered is the diffusional flux occurring from high concentration regions towards regions of lower concentration this is not the only possible scenario. Rigorously, the driving force behind diffusional fluxes is the free energy reduction of the system. To be still more precise, diffusion occurs from areas of high chemical potential to areas of lower chemical potential.
Nevertheless, it can be shown that whenever the atomic mobility is sufficient, and in absence of composition differences or other factors which could cause a more important flux, a high temperature gradient also causes a net flux of interstitial elements towards higher temperature regions. This effect is produced because, on the one hand, as regions at higher temperature are in a state of lower saturation, as they feature greater solubility, and therefore they would have a lower chemical potential than regions at higher saturation in the same temperature conditions. On the other hand, the flux towards high temperature regions is encouraged by the increase in atomic mobility as the temperature increases.
The presence of hydrogen in metallic alloys, especially in steels, is due to several reasons, from the presence of humidity in the raw materials or equipment or the decomposition of compounds present in the later, as well as actions performed during the alloy casting and refining process, for example those where hydrogen is blown through the molten metal with the aim of eliminating other elements, with the final consequence that some fraction of the hydrogen used remains dissolved in the molten metal.
During the casting process, heat extraction from the metal occurs through the walls of the mold and from the free surfaces of the cast metal.
In this manner, the cast metal generally cools from the surface to the core of the casting. That is, the casting's core remains at higher temperature than its surface, producing an increasing temperature gradient from the surface towards the core.
This marked temperature gradient, at temperatures at which interstitial elements such as hydrogen still feature a high mobility, produces a flux of interstitial elements towards the casting core, due to its higher temperature and greater capacity to dissolve said elements with respect to the adjacent regions which are at lower temperatures.
This diffusive flux tends to concentrate the total content of the interstitial element in question in the core region of the casting.
Due to the damaging effect of hydrogen in the mechanical properties of the components produced, traditionally different systems have been used to eliminate it.
These systems can be divided into two families: The use of certain additions during the refining process or the exposure of the molten metal to a reduced pressure.
The first of these methods consists in the addition of refining elements or substances that would combine with hydrogen (or other elements) and form insoluble substances that could be then eliminated during the refining process.
The second system consists in exposing the molten metal to an atmosphere with reduced pressure, as hydrogen solubility in the molten metal is function of pressure as well as of temperature and crystalline structure.
This second system produces a better hydrogen elimination rate, although at the expense of a large increase in the investment for the necessary equipment. For its part, the first system entails a much smaller investment, but it has also a lower hydrogen reduction rate, so that it is much less effective. Furthermore, this first system has the added issue that implies the modification of the alloy composition.
Therefore, the need is clear for a method which reduces interstitial elements, particularly hydrogen, in a casting process, without the modification of the alloy composition (with the exception of interstitial elements themselves) and furthermore, without requiring a large investment such as in the case of vacuum casting and refining.